In the prior art, constructions are known in which, in the tail threading in web slitters, devices are used which are based on various blowings and suctions. It has been noticed though that such blowing and/or suction devices are not sufficiently reliable for taking the leader end of the web of fill width from the paper reel and for passing the web of full width from the unwind station to the slitter.
At present, in paper mills, of the various stages of processing, attempts are made to eliminate those stages that require an abundance of manual operations. One problematic stage is the tail threading stage, which often requires the work of several persons and is required in connection with finishing devices, winders, cutters, etc. Also, in paper machines, high running speeds are sought, in which case, in a paper finishing device, the web threading must take place as quickly and reliably as possible.
In the prior art, constructions are known in which the threading of the paper web through a finishing device has been accomplished while the web has a full width, but the control of the web tension during the tail threading has proved problematic. The tension of the web has an important effect on a successful threading, for the following reasons:
Sufficient tension alone can guarantee a successful slitting during threading. PA1 Uneven tension has the consequence that the web is torn at tension peaks or that the overall tension must be kept very low, in which connection a part of the web remains even non-tensioned, i.e., slack. PA1 A slack web portion may enter into contact with structures placed near the winding path, in which connection the web can be torn. PA1 Adequate tension facilitates the formation of a good roll bottom at the beginning of winding.
In the prior art, constructions are also known in which a web of full width is attached from its edges and passed from the unwind station to the slitter. In one commonly used construction, it has proved to be a problem that the web tends to be torn because a highly uneven tension is formed in the web.
With respect to the prior art, reference is made to Finnish Patent 91,629 which describes a reel slitter whose function is to slit a web of paper machine width in the longitudinal direction in a slitting or cutting station. In the arrangement known from this patent, a pull-in bar is used which can be passed through the machine by means of a chain and to which the initial end of the web can be attached. This prior art reel slitter comprises two support drums, on whose support the webs are wound. The pull-in device provided with a pull-in bar feeds the component webs onto a first one of the support drums. Further, a transfer device, which receives the non-adjacent component webs after their separation from the pull-in bars, feeds the webs by means of a suction tube onto a second one of the support drums. It is one of the drawbacks of this prior art construction that, since the threading bar is pulled from its ends, during bending it produces an uneven distribution of tension in the web (compare FIG. 5A), from which it follows, among other things, that the web can be readily torn at the edges where the tension is high. Owing to the risk of tearing, the threading must be carried out with a very low overall web tension, and thus, in the direct beginning of winding, the desired web tension cannot be achieved. Furthermore, a slitting during threading can tear the web because of the low tension.
It is a further problem in this prior art construction that the web tension measurement devices of the slitter-winder cannot be used until the threading bar has by-passed the web guide roll which measures the tension of the web. In this construction, the fastening of the web to the threading bar is carried out manually. As such, aligning the leader end of the web on the threading member in the longitudinal direction of the slitter-winder so that the end of the web is exactly at the same location as the web is difficult during winding and inaccurate, i.e., it is difficult to place the ends of the component webs on the winding drums in the correct locations in the axial direction of the winding drums.